The present application relates to a communication apparatus, and a method of controlling the antenna characteristics.
In recent years, mobile telephones and mobile information terminals mounted with a non-contact type IC (Integrated Circuit) card or a non-contact type IC chip, or communication apparatuses, information processing devices, or the like mounted with a reader/writer function for non-contact communicating with the non-contact type IC card are being widespread used. Such devices or equipments are hereinafter sometimes referred to as a non-contact communication apparatus. The reader/writer and the non-contact type IC card can perform proximity communication by respectively using a carrier wave of a specific frequency (e.g., 13.56 MHz). For instance, when a command for causing the non-contact type IC card to execute a predetermined process is transmitted by the reader/writer, the non-contact type IC card executes a process corresponding to the received command, and returns the execution result as a response signal.
In this case, the non-contact type IC card can transmit the signal using a modulation technique referred to as load modulation of performing modulation on the carrier wave by changing the load of the antenna according to the transmission data. However, the non-contact communication using the load modulation forms a non-communicable region (so-called NULL) in relation to the distance (hereinafter referred to as non-contact communication distance) between the reader/writer and the non-contact type IC card. Thus, development of a technique of preventing the formation of such non-communicable region is desired.
In this regards, a technique related to an information processing device for reading out information recorded on the non-contact type IC card is disclosed in Japanese Patent Application Laid-Open No. 2004-240716 (hereinafter referred to as document 1). This technique relates to a technique of, after transmitting a command to the non-contact type IC card, changing an output impedance of a transmission unit according to the information responded by the non-contact type IC card with respect to the relevant command. The document 1 also describes a technique of changing the output impedance of the transmission unit when the response of the non-contact type IC card is not received. This technique is contrived in anticipation of the effect of avoiding the formation of the null, which is formed according to the non-contact communication distance.
In another example, a technique related to a communication apparatus for performing a short distance communication with the reader/writer is disclosed in Japanese Patent Application Laid-Open No. 2006-238398 (hereinafter referred to as document 2). This technique has one feature in estimating the distance between the communication apparatus and the reader/writer, and shifting the tuning frequency according to the estimated value. This technique also detects the reception level to estimate the distance between the communication apparatus and the reader/writer. Furthermore, this technique relates to a technique of using a capacitor for tuning the frequency of the signal, and selectively switching the tuning frequency by switching the connection of other capacitors connected in parallel to the relevant capacitor with a switch. This technique is contrived in an anticipation of the effect of avoiding the formation of the null, which is formed according to the non-contact communication distance.
In another example, a technique related to a reader/writer for performing a short distance communication with the communication terminal through a carrier wave is disclosed in Japanese Patent Application Laid-Open No. 2007-74153 (hereinafter referred to as document 3). This technique has one feature in receiving a synthesized wave of the carrier wave and the modulation signal from the communication terminal, and performing a control to attenuate the carrier wave according to the detection result of the modulation component contained in the synthesized wave. The document 3 also describes a configuration of arranging an auxiliary antenna for outputting a signal that counteracts the carrier wave. This technique aims to avoid the formation of the null corresponding to the non-contact communication distance.
In another example, a technique related to a communication apparatus for performing a short distance communication with the reader/writer, similar to document 2, is disclosed in Japanese Patent Application Laid-Open No. 2006-279813 (hereinafter referred to as document 4). This technique has one feature in estimating the distance between the communication apparatus and the reader/writer, and shifting the tuning frequency according to the estimated value. This technique also relates to a technique of detecting the impedance of a resistance connected to a means for tuning the frequency of the signal, and estimating the distance according to the change in impedance when estimating the distance between the communication apparatus and the reader/writer. This technique aims to avoid the formation of the null corresponding to the non-contact communication distance.